Electron discharge apparatus for amplifying ultra high frequency waves



July 31, 1951 s. RAMO 2,562,738

ELECTRON DISCHARGE APPARATUS FOR AMPLIFYING ULTRA-HIGH FREQUENCY WAVES Filed May 4, 1949 Fig.1.

/27 f K a I l/ F/ II I g I Irfiventor: Simon -I'Qaxrno,

His Attorneg.

Patented July 351, i951 ELECTRON DISCHARGE APPARATUS FOR AMPLIFYING ULTRA HIGH FREQUENCY WAVES Simon Ramo, Los Angeles, Calif., assignor to General Electric Company, a corporation of New York Application May 4, 1949, Serial No. 91,398

1 Claim. (Cl. 315 -39) My invention relates to ultra high frequency apparatus and has particular reference to electron discharge apparatus for amplifying ultra high frequency electromagnetic waves.

It is fundamental that forces may be exerted on an electron by either an electric or a magnetic field, and consequently either of these means may be used for controlling the path of an electron in space. Hence, in electron discharge apparatus an electron stream or beam may be deflected or accelerated by the use of either an electric or a magnetic control field.

Electric fields have been'commonly used for control purposes in discharge apparatus operating in the high frequency range of the radio spectrum although static magnetic fields are often employed to set the conditions for opera tion, as in discharge apparatus of the magnetron type. It has been easier to build up a high intensity of electric field in the path of the electrons than to create a magnetic field of suflicient intensity to exert an equalforce on the electrons with a given amount of high frequency power. The reason for this is apparent when it is recalled that the force exerted upon an electron by a magnetic field is proportional to the velocity of the electron whereas the force exerted by an electric field is independent of the electron speed. Although in a simple plane wave of any frequency in free space the ratio of magnetic to electric field amplitudes is such that eachexerts equal force on an electron moving with speed of light, the magnetic field at low electron velocities is relatively ineffective.

' electron discharge apparatus has been developed to operate at higher frequencies the velocity of the electron stream has been increased, primarily to reduce the transit time of the electrons from the cathode to the anode. With operating frequencies in the range of thousands of megacycles per second, electron streams having a velocity in the order of to 6 that of light may be profitably employed. At such velocities a magnetic field force is of the same order of magnitude as the electric field force, and control by a high frequency magnetic field becomes feasible. Several advantages result from the use of magnetic field control, the most important of which is that the force always acts perpendicular to the electron motion. Thus very little work need be done by the controlling force, resulting in minimum input loading of apparatus using this principle. y

' It is an object of the present invention to pro-ivide electron discharge apparatus utilizing a high frequency magnetic control field and offering the advantage of low input loading.

Electron discharge apparatus of the magnetron type has presented a very simple and effective means for generation of ultra high fre quency waves. In attempting to apply the magnetron principle to ultra high frequency amplifying apparatus, limitations in design have resulted from the difficulty of keeping the input and output systems electrically separate. Because the rotating space charge of a magnetron interacts with the electric fields presented at the gaps of the anode cavity resonators, a magnetron tends to oscillate. Efforts made in the past to overcome this effect have gone so far as to mechanically divide the magnetron anode trans-' versely of its axis and impose an axial component on the space charge so that the space charge moves axially from 'an input section of the anode to a separate output section. While such changes in the magnetron structure make it suitable for use as an amplifier, they also make it undesirably complex.

Therefore it is a further object of this invention to provide a simple ultra high frequency amplifier construction of the magnetron type.

Broadly described, my invention is an ultra high frequency discharge apparatus having a high velocity electron stream or beam which is periodically radially deflected by the magnetic field of an ultra high frequency input signal, and having an output circuit tuned to the same frequency which is periodically excited by the electron stream in accordance with its deflection. The input means comprises a hollow metallic resonator having apertures 'of an excitation gap in the trajectory of the electron stream so that the electron stream may be deflected by the magnetic field of the standing wave produced in the resonator. To provide the most eificient magnetic deflection and to prevent the loading of the input source by the presence of an electric field in the path of the electron stream, the electron stream is exposed to a region .of maximum magnetic field amplitude and a corresponding electric field node of the standing wave. This region may be determined by conventional means, such as by using as the input resonator a section of wave guide short-circuited at a distance from the electron beam apertures or gap equal to one-half the wave length at the resonant frequency. Thus when the input resonator is excited, the electron stream will be deflected periodically by the magnetic field as it varies from its minimum to maximum; intensity.- Sin ilarly, the output resonator may comprise a section of wave guide having apertures or an excitation gap and short-circuited a distance therefrom equal to one-quarter of a wave length at the resonant frequency. As the electron stream is periodically deflected through the resonator apertures or closer to the excitation gap, the output resonator is accordingly excited. The resonator can be arranged in the path of the electron beam in a number of Ways so as to be responsive to the deflection of the electron stream. One arrangement is to locate the aperture of the resonator slightly aside from the path of the unmodulated electron stream, so that only a part of the deflected stream passes through the output resonator during each period. I may also position the resonator so that the electron stream passes near an excitation gap, the resonator being periodically excited by the electron stream as it is deflected to a path closest to the excitation gap.

In one embodiment of my invention I employ amagnetron type of discharge apparatus as an ultra high frequency amplifier. Using a conventional magnetron anode block having a central circular opening with a cathode extending through the center of the opening, the rotating space charge of the magnetron is modulated by an ultra high frequency magnetic field. A radial input slot in the anode block defining a gap at the central anode opening and extending into the anode block a distance equal to one-half wave length at the frequency to be amplified provides the means for magnetic control. With an input signal coupled to this radial slot, the slot resonates and the standing wave established therein has maximum magnetic field amplitude at the gap, the electric field being at a node in that region. When a static magnetic field is set up parallel to the cathode and an electric field imposed between the cathode and the anode, a rotating space charge consisting of electrons rotating around the cathode within the anode opening is established. The ultra high frequency magnetic field at the gap extends axiall along the gap and exerts a more or less radial force on the rotating space charge. Since the magnitude of the magnetic force varies with the amplitude of the input signal exciting the half-wave slot, the space charge is modulated accordingly and current is induced in a conventional output resonator located in another part of the anode. As an output resonator a second radial slot defining a gap at the central anode opening and extending into the anode block a distance equal to one-quarter wave length at the resonant frequency is employed. The output gap is excited by the rotating space charge to produce a standing wave whose electric field amplitude will be a maximum at the gap, in the manner of a conventional magnetron anode cavity. The amplitude of the excitation in the output resonator varies with the radial deflectionsof the modulated space charge. Conventional output coupling means are used. An important aspect of this embodiment of my invention is that since the gap of input resonator presents a very low impedance to the space charge it modulates, the input circuit is very insensitive to the resultant space charge waves, thus permitting the device to function successfully as an amplifier.

The features which are desired to protect herein are pointed out with particularity in the appended claim. The invention itself, together with pertinent objects of the advantages thereof may best be understood by reference to the following description taken in connection with the drawings in which Fig. l is a vertical section of a magnetron type of amplifier embodying my invention, and Fig. 2 is a transverse sectional view of the anode structure of the embodiment shown in Fig. 1.

Referring now to Fig. 1 of the drawing, I have there illustrated my invention as embodied in a magnetron type structure generally disclosed in Patent 2,412,824, issued December 17, 1946, to Elmer D. McArthur and assigned to the assignee of the present application.

The electrodes of the discharge apparatus are shown as enclosed within a cylindrical container I made of a ferro-magnetic material with the ends of the container closed by flanged members 2 and 3 which are welded or otherwise hermetically sealed to the inner surface of the cylinder l. Within the container in a central region I provide an anode structure 4, shown also in Fig. 2. The annular anode structure 4 is preferably formed of a disk of a suitable metal, such as copper. The disk should be of an appreciable thickness, being equal or greater than a dimension equivalent to /2 wavelength at the frequency to be amplified. Within the anode structure 4 is provided an enlarged circular central opening 5 in which a cathode structure 6 is centrally positioned. The cathode 6 is shown as comprising a flanged sleeve which is preferably coated on the outer surface with an electron emissive material such as barium oxide. A cathode heating element 1 is located within the sleeve.

In order to modulate the space charge which is established in the central opening 5 between the cathode and the anode, I provide Within the anode structure 4 a plurality of circumferentially positioned space resonant regions or cavities about the central cathode opening. As further shown in Fig; 2, the input cavity is a radial slot 8 extending radially from a central opening 5 into the anode disk for a distance equal to one-half wave length at the frequency to be amplified. A second output cavity responsive to the modulated space charge may take the form shown as radial slot 9, which extends radially from the central opening 5 into the anode block a distance equal to A wave length at the amplified frequency. The slots thus formed constitute, respectively, half-wave and quarter-Wave resonators shorted at their outer ends by the anode metal and defining gaps at the central anode opening.

Conventional electrode means for exciting the half-wave slot, and for deriving energy from the quarter-wave slot may be utilized. For example, a loop l0 extends into the rectangular opening 8 and constitutes an extension of an inner conductor of a concentric transmission line H, comprising an inner conductor I2 and an outer tubular conductor l3. In like manner, energy may be derived from the quarter-wave slot 9 by a loop ll which constitutes an extension of an inner conductor l6 of a concentric transmission line 15 having an outer tubular conductor H.

To provide a magnetic field for the discharge apparatus, I furnish within the container l magnetic pole pieces I8 and [9 which are directed axially of the container and extend in close proximity to the upper and lower surfaces of the anode structure 4. Where it is desired to employ permanently magnetized pole pieces, these members may be constructed of a magnetizable material having a high coercive force and a high energy factor, such. as alloys of the class including aluminum, nickel and cobalt. A low reluctance path between the bases of the magnetic poles is and I9 is obtained by seating the pole pieces upon relatively thick disk-like members 26 and 2! constructed of a ferromagnetic material such as steel.

In order to position and support the cathode structure, the magnetic pole pieces 18 and I9 are provided, respectively, with longitudinally extending channels 22 and 23 which may be of circular cross-section. One end of the cathode 6 may be supported in the desired central position by means of an insulator 24 which closely engages the walls of channel 22 in the magnet is, and the other extremity of the cathode structure may be similarly supported by means of an insulator 25 which is positioned in the upper end of channel 23. Channel 22 in pole piece is may also be employed for supplying current to the cathode heater 1. For example, a pair of conductors 25 and 21 electrically insulated from each other and from the walls of the channel 2| serves as a means not only for supplying energy to the heating element of the cathode, but also as a means for obtaining an external connection to the cathode 6.

The discharge apparatus may be evacuted through channel 23, which is terminated in a tubulation 28. The tubulation is sealed after evacuation.

To operate the discharge apparatus as an amplifier, a unidirectional potential is first impressed between the anode structure 4 and the cathode 6. Due to the presence of the static axial magnetic field produced by pole pieces [8 and IS, a rotating space charge will be established be tween the outer surface of the cathode B and the anode structure i. This motion may bedescribed generally as following a spiral path, the angular velocity of the electrons in the space charge being a function of the static magnetic field intensity.

To energize the input cavity 8 the input transmission line H is connected to a source of ultra high frequency to be amplified. A standing Wave in the half-wave slot 8 is thereby established. The amplitude of the magnetic field of the wave is at a maximum at the closed end of the slot and is again a maximum at the gap which is a half-wave distant. The electric field on the other hand has zero amplitude at the closed end of the slot and again at a node at the gap. While a minimum impedance is presented to the rotating space charge at the gap of slot 8 due to the small electric field, the magnetic field exerts a more or less radial force on the electrons which pass it, thus radially deflecting the electron stream. If the electron velocity is of the order of several thousand volts or higher, so as to be, for example, in the range of to that of light, the forces exerted by the high frequency magnetic fields will be comparablev with the forces that electric fields would have exerted at the gap if the more conventional quarter-wave resonator were used.

Since the output resonator comprising radial slot 9 has a quater-wave length the impedance presented to the space charge is very high, the electric field amplitude being at a maximum at the gap. Thus a current is induced in the slot 9 whose amplitude varies according to the modulation of the space charge defined by the radial deflection at the gap of slot 8. The usefulmodulated energy thus amplified is derived from the slot 9 by means of pick-up loop I l and transmitted through transmission line I5 to be utilized as desired.

It is obvious that without departing from the scope of my invention, that resonant circuit means equivalent to the input and output cavities described above may be substituted, provided a maximum magnetic field and minimum electric field are presented at the anode gap for the input means and provided that the output circuit is arranged to be excited by the periodically deflected electron stream. Any resonant input slot an even number of quarter-wave lengths long and corresponding any resonant output slot and odd number of quarter-wave lengths long would be obvious substitutions.

While I have shown and described my invention as applied to a particular form of magnetron, it will be obvious to those skilled in the art that other changes and modifications may be made without departing from my invention, and I, therefore, aim in the appended claim to cover all such changes and modifications as fall within the true spirit and scope of my invention.

' What I claim as new and desire to secure by Letters Patent of the United States is:

An ultra high frequency amplifier of the magnetron type comprising a cathode and an anode structure having a central opening through which said cathode extends, means for establishing a static magnetic field which with an electric field applied across said anode structure and said cathode establishes therebetween a rotating space charge, said anode structure being slotted to define'a plurality of spaced space resonant slots therein resonant at the frequency of the wave to be amplified and extending radially from said central anode opening into the anode structure, one of said slots extending a distance equal to one-half the wave length at said frequency, input means coupled to said slot for producing therein a standing wave at said frequency having a maximum magnetic field amplitude at the central anode opening, a second of said slots extending a distance equal to onefourth the wave length of said frequency in order to be excited at said frequency by said rotating space charge in response to the periodic radial deflection of said rotating space charge produced by the magnetic field at the gap of said first slot.

SIMON RAMO.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,423,716 McArthur July 8, 1947 2,429,291 Okress Oct. 21, 1947 2,446,825 Gurewitsch Aug. 10, 1948 

